1. Field of the Invention
This invention relates to a toner used in image forming processes such as electrophotography, electrostatic recording, electrostatic printing and toner jet recording.
2. Related Background Art
In recent years, copying machines and printers are severely sought to be more compact, more light-weight, more high-speed and more high-reliability because of demands for energy saving, office space saving and so forth. As the result, hardware has come to be constituted of simple components in various aspects, and on the other hand the performance demanded of toners has come higher. That is, it is coming to pass that any superior hardware can not be provided unless improvements in performance of the toners can be achieved. In particular, what is required of the performance of toner is transfer performance. Improvement in this transfer performance of toner can bring advantages that toner consumption can be reduced, image quality can be made higher and a toner collection system can be made simple.
Accordingly, as one of methods for improving the transfer performance, it is recently prevalent to make the shape of toner base particles closely spherical. For example, it is known to use polymerization toner base particles produced by polymerization such as suspension polymerization or emulsion polymerization, or to make pulverization toner base particles spherical in a solution, to make them spherical by the action of hot air (Japanese Patent Application Laid-open No. 2000-029241) and to make them spherical by applying a mechanical impact force (Japanese Patent Application Laid-open No. H07-181732). These techniques are very effective means to improve transfer performance of toners, but each have various problems. In the case when the polymerization toner base particles are used, a release agent is enclosed in toner base particles, and hence the release agent may come out to toner particle surfaces with difficulty unless a pressure is appropriately applied to the toner at the time of fixing, resulting in a poor fixing performance. Also, in the case when the pulverization toner base particles are made spherical by applying a mechanical impact force, the release agent contained in toner base particles may more easily melt and come to toner particle surfaces because of heat, as they are made spherical more and more. This may adversely affect electrophotographic performance. That is, the release agent having melted and come to toner particle surfaces may make the toner have poor fluidity, or may make toner particles themselves and/or toner particles and carrier particles highly adherent to damage transfer performance. Accordingly, in making spherical the toner base particles to which the release agent has internally been added, it is important to select toner constituent materials capable of being made readily spherical, or to restrain any effect of heat history on toner base particles during the step of spherical treatment.
In order to manage such toner base particles to which the release agent has internally been added, having been improved in sphericity as stated above, it is proposed as disclosed in, e.g., Japanese Patent Application Laid-open No. H10-097095 to control average circularity and circularity distribution of toner base particles to control transfer performance and chargeability of a toner. However, in what is disclosed in Japanese Patent Application Laid-open No. H10-097095, a styrene-acrylic resin is used as a binder resin to be used in the toner, and no reference is made as to a color toner composed chiefly of a polyester.
In the step of making toner base particles spherical, in particular, in the step of making toner base particles spherical by applying a mechanical impact force, very small-sized fine powder (ultrafine powder) tends to form, and such ultrafine powder may re-agglomerate with the toner base particles having been made spherical. Hence, even if such toner base particles are subjected to the step of classification after the step of making spherical, it is difficult to remove this ultrafine powder from the toner base particles, and the ultrafine powder tends to be mixed into products. This ultrafine powder may be charged in excess in the step of development, and may adhere electrostatically to a sleeve, or a carrier in a two-component development system, to contaminate them to cause faulty charging of the toner fed afterwards.
Proposals to control the quantity of such ultrafine powder are also made in a large number. For example, as disclosed in Japanese Patent Applications Laid-open No. H10-232507 and H11-149174, a method is proposed in which the ultrafine powder is driven into toner particle surfaces by applying a mechanical impact after the step of classification. However, because of agitation at a developing zone and in a toner container throughout long-term running, the ultrafine powder driven into toner particle surfaces may come off to affect developing performance adversely.
On account of the foregoing, it has been sought to provide, in a color toner containing a release agent, a color toner having superior transfer performance, running performance and charge characteristics.